Most screen biographies work hard to establish realism. 20,000 Days on Earth, a fittingly ostentatious account of a day in the life of the rock poet Nick Cave, instead takes on the curious task of fictionalising documentary.

While the film is presented as a single day – beginning as Cave wakes up and ending at night as he waxes philosophical while staring into the water at Melbourne’s Brighton beach – it was clearly shot over much longer than 24 hours. Conversations, while earnest and free flowing, are contrived, not dissimilar to monologues from a Don DeLillo novel or a Jim Jarmusch film.

Like some of the most compelling documentaries in recent years, including Sarah Polley’s Stories We Tell (2012) and Casey Affleck’s I’m Still Here (2010), 20,000 Days on Earth is an exercise in blurring truth and fiction, centred on a performer clearly sceptical of documentary as a factual medium.

Directors Iain Forsyth and Jane Pollard shape the film as a low-key excursion into rock celebrity and alternative artiste mythology. There’s lots of talking, lots of references to drug-addled pasts, a string of disconnected moments and bursts of music and spoken word. 20,000 Days on Earth is an odd kind of vanity project, like something in which Joaquin Phoenix or a younger Bob Dylan might have starred.

Scenes in which Cave partakes in on-camera interviews, in which he is gently probed for his thoughts on topics as serious as the death of his father and his greatest fears (losing his memory and uncontrollable weather) could have been filmed on a PR junket. There is nothing extraordinary about them, but the context is shrewdly rearranged with a small change: Cave’s interviewer is a shrink (or at least plays one) and not a journalist.

In two separate scenes that play like a cross between David Cronenberg’s Cosmopolis (2012) and Jerry Seinfeld’s web series Comedians in Cars Getting Coffee, Ray Winston and Kylie Minogue randomly appear in Cave’s 1980s Jaguar XJ (black, of course) to chew the philosophical fat before disappearing as quickly as they arrived. Like the comers and goers who jump into Robert Pattinson's limo in Cosmopolis, they serve no obvious purpose other than to share and extract deep thoughts. In any other kind of film these cameos would feel like celebrity name dropping, stars to put on a poster. Here they're like a natural extension of Cave’s mysterious existence; leftovers from his dreams, perhaps, or apparitions he willed to life.

Forsyth and Pollard pull together a polished-looking product. There is a brooding but beautifully manicured quality to the film's aesthetic, as if Cave’s swooning counter-culture style played a part in dictating the mood. He is a strange mixture of introvert and extrovert, someone you can keep looking at but never get any closer to understanding. The film is less complicated. There’s nothing extraordinary about it, but it’s an entertaining curio.

A film about Nick Cave heralds the start of this year's festival on 4 June, with a Robert Altman retrospective and a Richard Linklater movie that took 12 years to film among the highlights. Here's a rundown of the films you shouldn't miss, writes Lynden Barber